Awhile ago, they changed live chess so that even premoves take some fraction of time off your clock.

I am aware of that but I don't use premoves. I'm talking about playing a move that gives rise to checkmate and seeing it "stick", then a fraction later, the "You've lost on time" notification appears. With the board still showing checkmate.

I have had precisely the same problem! I do believe this is a real problem with the chess.com interface.

just as context - time is measured in 2 places: in the browser, and at the server. the difference between them is lag.

so, say that we send you a move at time 0:00.00 from the server. you receive it at 0:00.50 in your browser. because that time is close, we consider that lag as normal and the time discrepency is eaten by your opponent. meaning, neither clock runs during that time.

then let's say you make a move at 0:01.50 (1 second later), but the server doesn't receive the move until 0:03.50 (2 seconds after you move). who is supposed to absorb that lost time? you? your opponent? the length of the game?

what we have decided after looking at millions of chess moves is that more than 95% of chess moves have less than 2 seconds lag (with most being less than 200ms). if that is the case, the game length absorbs the lag (meaning, neither timer runs for that imperceptable amount). if it takes more than 2 second of lag, then the lagging user absorbs the difference. so if it takes your moves 3.5 seconds to reach the server, you lose 1.5 seconds (because we smush the 2 seconds out as lag).

so, say on your final move you lag 4 seconds, and had only 1 second on your clock, you are going to lose on time. :/ but again, that only happens on less than 5% of moves, and unfortunately, happens mostly to people on weak internet connections and/or far away from our servers.

we are looking into adding 3 new server locations this year to help with this.

Oh, so that's why it's possible to lose on time even though my last move shows up on my side.

Why not use something similar to time stamping? I'm no programmer, so maybe the implementation is hard, but the idea of it is very simple. And at the end of the game, when their server reads 0:00 for you, there would be a check that pings the player for their last move and checks the time stamp before declaring the game lost.

It seems just the same however fast one's computer is. In blitz over the board it shouldn't be too difficult to play, say, 8 moves in 10 seconds. Realistically, it should be possible to play 7 moves in 5 seconds, just about. Over the board, I've won plenty of games where I've been down to less than a minute and needed 20 moves to win. When I was good I used to turn down draws when I had two minutes left and my opponent had 15 mins if I thought the position wasn't blocked and I had a positional advantage. No, the point is there's something wrong with the programming here if it takes over a second a move. So the problem is not that I'm a slow mover but that I',m a fast mover and the programming doesn't keep up. I've been in a position where I've had the cursor poised over my piece and it's been a really short move to checkmate, with two seconds left showing on the clock. That realistically gives a minimum of a second for something that should take 0.4 of a second. Yet in such a situation I've still lost, which makes a mockery of the times shown. If someone has a fast computer with no problems, any lag is programming error at the Chess.com end, imo.

There are other silly errors, like having pop ups on the screen where one challenges an opponent at blitz so that when they pop up, they obscure his rating. All in all, they bombard people with invitations to pay yet on the evidence of what is available, it seems like it's going to be a waste of money.